So, explain to me how the slave drive is any safer or less likely to fail that the system drive?
You still relying on a single piece of hardware located in the same box as the other drive, connected to the same power source, etc. to maintaina single copy of your most important data. This only protects you from lossing files in the event of the system drive failing, but you are still just as vulnerable to drive failure.
There is no substitute for regular backups and the offsite backup options available today are inexpensive, secure, and have a very low system impact on modern PCs. Barring that a regular backup to an external drive that does nothing more than copy the files to another drive is still a better solution than just placing the files onto a different drive. There are tones of free utilities to do this or you can even look up how to use the xcopy command, create a batch file, and setup a schedule to run it every night.